A single 4 gauge wire from the battery in the trunk probably isn't going to be big enough. You're likely to have starting issues with that. The longer the run of cable, the bigger the size must be to overcome the resistance in the extra length. I run a double 2-gauge cable (soldered together both cables in a jumper cable set). Many people run a 1/0 welding cable. Just a heads up. If you run out of room for big cable lugs on the fender-mounted starter switch, you can also run either the battery hot or the alternator output to the hot side of the battery.
Lastly, if the car's not overloading the twin 10 gauge wires now, it's unlikely to with the simple addition of a larger alternator. Alternators don't simply 'put out' the extra current they have the capacity for. The current is drawn from them by the accessories. So whether you have a 65A alternator, or a 150A alternator, if the accessories/electrical needs total 40A, that's all the alternator puts out.